The first thing to do is strip it down your chassis, and separate the metal parts into the different types (see image 01). For the SRB (Special Racing Buggy chassis: Rough Rider, Sand Scorcher, Ford Ranger, the Super Champ & the re-release versions) these will be brass, bronze, possibly metal bearings, a lot of aluminium parts, lots of "pot metal" (aluminium/zinc alloy) castings, Bright Zinc Plated Steel, Cadmium Passivated Steel, Blued Steel and what looks like Silver Steel ... metallurgy purely by the naked eye is a bit suspect, but the parts are of a sufficiently different look and feel here that it's ok to separate them like this.

If you're working on a different chassis, the ideas are mostly the same, just the proportions of materials will change - you may see some stainless steel or chrome plated steel in there too.

Brass Parts

(e.g. Universal Joints, shock absorber eyes, etc) are best cleaned in a ultrasonic tank with a little washing up detergent e.g. Fairy liquid (I like lemon flavour the best) and a lot of concentrated metal cleaner (e.g. Sea Clean) - cycle time will depend on the concentration, but between 10 & 30 minutes should suffice. Brass parts should already be pretty shiny by this point (image 02).

After the parts come out of the tank, they should be rinsed with clean water and dried - I use a cloth to get the worst off, then use a hairdryer to heat the bits enough so that anything left in the nooks & crannies will evaporate off.

Any remaining additions (dirt, sealant, threadlock etc) can be scraped off at this point, but what to do with tarnish (e.g. image 03) is open to question ... I used to religiously polish everything with Autosol - but that does result in a finish that is brighter and shinier than factory (see top row, image 04). On balance I think it's probably better to leave the bulk of the parts as they came out of the ultrasonic tank, and just polish the bits that show (salmon coloured) oxidation. An alternative is to use a fibreglass pen (see image 05).

Brass parts should not be confused (or mixed in an ultrasonic tank) with ...

Bronze Parts

(e.g. the 1150 size gearbox & front wheel bushings - see image 06) might look a similar colour to brass (an alloy of copper and zinc), but are chemically different to bronze, which is an alloy of mostly copper, but also tin, aluminium, or even phosphorus (as in "phosphor-bronze bearings"). The practical upshot is that if Bronze and Brass parts are mixed in an ultrasonic tank (especially with a metal cleaning solution), the brass parts will come out with a soft, matt pink layer plated on. This can be polished off, but it's easier if you don't have to.

Apart from separating the two types (and discarding the solution afterwards), Bronze parts can be treated the same way as Brass, above.

Metal Bearings

Metal bearings aren't part of the original kits but still can often be found in a restoration project. I'd suggest these are best initially flushed with a solvent (e.g. Isopropyl alcohol) or penetrating oil (e.g. WD40) rather than going in the ultrasonic tank, then placed on something plastic & oil dripped on, then left overnight. Pressing daily on to tissue will draw that oil out - repeat until it comes out clean (image 07).

Alternatively (or it they sound gritty or feel notchy or loose), just replace them - bearings are comparatively so much cheaper than they used to be in the 1970s, 80s or even 90s that unless you know they're genuine Tamiya items or hold some other special significance, it probably isn't worth the trouble.

The rollbar can be treated in a similar manner - minus the drill of course. Try and keep the sanding in the same direction for a "brushed" look. The chassis reinforcing plate will often be scratched/gouged - start with a very rough paper & work up to fine (images 08 & 09).

Restoring pot metal to a factory like finish is where it gets difficult and/or expensive. I've tried various methods including sanding, Dremelling with a wire brush and polishing with Autosol, but the best result I've tried so far is media blasting - using recycled glass grit and glass bead.

The problem with traditional sand blasting is that it leaves a finish that is very raw, white, sparkly, and handling it feels unpleasantly like fingernails down a blackboard - not at all like the factory finish. What's needed is an additional step, using glass micro beads - unfortunately you can't do it with just beads as it doesn't have the cutting ability to get the staining off to start with. (Note: I've also tried Walnut shell, but it just didn't work for me as it just doesn't have the cutting power in a small setup).

There's also the problem of getting things media blasted - as long as your item is fairly agricultural and you don't particular care what the finish is as long as its rust free, then it won't be too hard to find someone to do it at a reasonable price - but to take really good care of 30+ year old toy car parts? With the media and pressures specified? Even if you can find somewhere you're likely to be given an (ahem) "go away" price.

The alternative is to do it at home. The initial outlay for even a basic setup doesn't make sense if you're only ever going to refinish one set of SRB metal parts, but the more you do with the less painful it is - and besides, c. 250 GBP is the cost of one new re-release SRB kit, or two more basic plastic RC kits - and would you have such a problem with buying them?

Blasting cabinet: currently around 60 GBP on eBay;

Air compressor: a 2hp/24L unit can be had for as little as 80 GBP, but media blasting uses an awful lot of air & something like that I'd rate as "barely adequate": it'll be running almost continuously & need a long rest after half an hour;

Hose to join the two together: c. 15 GBP;

Adaptor: there are a silly amount of "standard" air fittings & the chances of yours all matching are slim: budget a extra fiver;

Old vacuum cleaner (for dust extraction): anywhere between free and c. 40 GBP if you have to buy a (good) cheap new one.

Method

(image 10 shows a typical SRB gearbox prior to stripping and ultrasonic cleaning): having cleaned the parts (see image 11 - note the sealant yet to be removed & the dark staining mentioned earlier) , bung up all the threaded holes with sacrificial fasteners. Tie together with wire any parts that are likely to escape through the grid in the blasting cabinet. (Note: twos make more sense than fours - visibility can be very limited in a blasting cabinet - see image 12).

I used the smallest nozzle that came with the tank & set the pressure on the compressor to 40psi before blasting with the recycled glass grit - doing all the pot metal parts took about half an hour (see image 13).

Changing media in a cheap tank is a bit of a pain, especially when going from a coarse media to fine when you really need to hoover out every last bit - but I don't see any way around it.

Loading the cabinet with glass bead & increasing the pressure to 50 psi saw the bits done in about 35 minutes. Scrub the bits with a toothbrush & fairy liquid (& water) to remove any last bits of media, then dry & remove the fasteners (see images 14 & 15).

This leaves a finish that is a little brighter and shinier than the factory finish, but will dull down a little as it ages.

Blued Steel (e.g. M3 cap head screws), Cadmium Passivated Steel (e.g. M4 bumper screws), BZP (Bright Zinc Plated) Steel (e.g. M4 Nyloc wheel nuts) and Silver Steel (at least I think that's what it is? - e.g. front suspension shafts, gearbox spindles etc) (left to right in image 16) should be separated out for cleaning in the ultrasonic tank (30 mins with detergent, rinse in clean water, dry, remove any remaining grot with a fibreglass pen) but can be mixed for the final step - add a few drops of oil, jiffle about so everything has a thin coat (image 17) - and dry off on a bit of rag before you use them.